Is this sentence really correct?

¡Hola Allison! The sentence you are refering to is a conditional construction where we use the imperfect of subjunctive in the "si" clause and the conditional tense of indicative in the second clause in order to express hyphotetical situations that have not yet happened. To use the conditional perfect in the second clause as you were suggesting, we should use the pluperfect of subjuntive in the "si" clause, due to the fact that using the conditional perfect means that the action is impossible to happen at the present time. However, our example "si viniérais, os haríamos una paella riquísima", is a bit more probable to happen but it is still unsure if it will. In your suggestion "si hubiérais venido, os habríamos hecho una paella riquísima", it is sure that they missed their opportunity of going to that place and the fact of cooking the paella for them will never happen at the present time. To sum up, the use of tenses in these type of conditional constructions is quite concrete and we cannot accept to use the imperfect of subjunctive + the conditional perfect in a conditional tense with "si" because the time frames are different and they don't make any sense. I hope that this explanation helps you to understand a bit better this concept. ¡Gracias! Silvia.